It is a modern and advanced transportation mode to take a container as a unit for transportation. Containerization has become the main trend of the international cargo transportation. Meanwhile, the uses of containers in smuggling firearms, weapons, drugs, explosives, even WMDs (Weapons of Mass Destruction) and RDDs (Radiological Dispersal Devices) have become an international public hazard troubling every government and interfering with the normal order of the international cargo transportation.
Since 9.11 in U.S., the U.S. government began to attach great importance to the potential risks of the cargo transportation, and worried more about WMDs and RDDs through containers into the United States. To guard against such risks, the U.S. Customs Office issued a “Container Security Initiative (CSI)” in Jan. 17, 2001, in which all the foreign ports with transactions directly linked to the U.S. port were required to be equipped with non-invasive X(γ)-ray scanning imaging equipments for performing ray-scanning inspections on the containers shipped to the U.S. One year after CSI being announced, there were 18 major ports in the world which joined the initiative and began to operate. In the background where the requirements for the international transportation safety are increasing, the World Customs Organization unanimously adopted a resolution calling for all the 161 members to develop a plan regarding the container security inspection along the mode of the CSI, i.e the container security inspection has become a world's common topic of concern.
The existing container X(γ)-ray security inspection equipments mainly aim at transmission imaging, i.e. to directly penetrate cargo using X-rays so as to obtain transmission images of all the articles covered by the path of the X-rays. The standard transmission imaging technology solves the “visualization” problem of containers, such that it has been widely used. However, such equipments are usually of the following disadvantages. First, information with a two-dimensional structure is vulnerable to the articles overlapping on the path of the rays. Second, density information is not involved. Third, material information is not involved either.
With respect to a demand for “prevention of smuggling”, a main countermeasure against the smuggling is to compare the item information listed in Customs Declaration with high-energy X-ray scanning images of containers, and then inspect whether the above both are matched. Here, the Customs Declaration is priori knowledge, and a standard X-ray transmission imaging technology may substantially meet the above demand. However, the proposal of the CSI makes the demand for the container inspection developing from inspecting of smuggled articles (“prevention of smuggling” for short) to inspecting of dangerous articles (“prevention of danger” for short). Since there are various types of dangerous articles, and they have no fixed shapes, i.e. there is no priori knowledge for articles in the container under inspection, it has already appeared difficult to meet the requirements for the container security inspection only relying on the standard X-ray transmission imaging technology.
The more accurate and efficient security inspection is possible only if more plentiful features of the object is acquired according to the characteristics of WMDs, RDDs and the other dangerous articles. A dual-energy technology uses two kinds of X-ray having different energy spectra to penetrate an object under inspection. Signals acquired under difference energies are processed to obtain the atomic number information of the material for the object. Thus, the security inspection level is efficiently improved to a certain extent by using this technology. It is desirable that the high-energy X-ray imaging container inspection system posses a material discrimination ability, which has become a hot spot of the international research in recent years.
The dual-energy technology is especially efficient when the energy of the X-ray is lower than 200 KeV, which has been widely used in luggage inspection. However, when the energy of the X-ray penetrating the container achieves several MeV, for different materials with the same mass thickness, such as C, Al and Fe, the attenuation over this energy spectrum has no great effect on the attenuation of the ray. Therefore, the material discrimination ability obtained by using the high-energy X-ray imaging is far worse, compared with the low-energy dual-energy X-ray technology. Even some experts in the field of the container inspection believe that the dual-energy imaging technology has almost no effect when the energy of the X-ray is beyond 200 Kev, thus this technology is not suitable for the container inspection system.